FIG. 1 is a diagram showing a set of submodules 6 of a modular multilevel converter 2 of the prior art. For three-phase input/output currents (having three phases φa, φb, and φc), the converter 2 has three conversion legs that are referenced by indices a, b, and c for the various components in FIG. 1. Each conversion leg comprises an upper arm and a lower arm (indicated by indices “u” for upper and “l” for lower), each connecting a DC+ or DC− terminal of the DC power supply network to a terminal of the AC power supply network. In particular, each of the legs is connected to one of the three phase lines φa, φb, or φc of the AC power supply network. FIG. 1 shows a set of submodules 6, in which each arm comprises a plurality of submodules SMxij that can be controlled in a desired sequence (where x indicates whether the arm is an upper or lower arm, i indicates the number of the leg, and j indicates the number of the submodule in the submodules in series in the leg). In this example, only three submodules are shown per arm. In practice, each lower or upper arm may have a number N of several tens to several hundreds of submodules. Each submodule SMxij includes an energy storage system such as at least one capacitor and a control member for connecting the capacitor selectively in series between the terminals of the submodule or for bypassing it. The submodules are controlled in a selected sequence in order to vary progressively the number of energy storage elements that are connected in series in an arm of the converter 2 so as to provide a plurality of voltage levels. In addition, in FIG. 1, vdc designates the voltage at the point where the converter is connected to the DC power supply network, these points being referred to as points of common coupling (PCC) as is well known to the person skilled in the art. idc designates the current in the DC power supply network, whereas currents iga, igb, and igc flow in the three phase lines φa, φb, and φc. In addition, each arm possesses an inductance Larm and each phase line includes an inductance Lf and a resistance Rf.
FIG. 2 shows a prior art submodule SMxij forming part of the FIG. 1 converter. In this submodule, each control member comprises a first electronic switch element T1 such as an insulated gate bipolar transistor (IGBT) connected in series with an electrical energy storage element, specifically a capacitor CSM. This first switch element T1 and this capacitor CSM are connected in parallel with a second electronic switch element T2 that is likewise an insulated gate bipolar transistor (IGBT). This second switch element T2 is connected between the inlet and the outlet terminals of the submodule SMxij. Each of the first and second switch elements T1 and T2 has an antiparallel diode, as shown in FIG. 2.
In operation, the submodule may be controlled to occupy two control states.
In a first state, referred to as the “ON” state, the first switch element T1 is open and the second switch element T2 is closed, so as to connect the energy storage element CSM in series with the other submodules. In the second state, referred to as the “OFF” state, the first switch element T1 is closed and the second switch element T2 is open so as to short circuit the energy storage element.
It is known that each arm, having a voltage vm across its terminals may be modeled by a modeled voltage source having a voltage vm across its terminals and of duty ratio that depends on the number of controlled submodules, and by a modeled capacitor Ctot connected to the voltage source. This model is shown in FIG. 3, in which there can be seen an arm and the resulting model. The reciprocal of the capacitance of the model capacitor Ctot is equal to the sum of the reciprocals of the capacitances of the controlled submodules, such that:
      1          C      tot        =            1              C        1              +          1              C        2              +    …    +          1              C        N            where C1, C2, . . . , CN are the capacitances of the jth capacitors.
Thus, the voltage vcΣ across the terminals of the modeled capacitor Ctot is equal to the sum of the voltages vcj across the terminals of the capacitors of the submodules in the arm (where j goes from 1 to N and indicates the number of the capacitor and thus of the submodule). In the present application, and by abuse of language, Ctot designates both the capacitor and its capacitance. By controlling the sequence with which the submodules are controlled so as to cause the number of energy storage elements that are connected in series to vary progressively, the energy of the modeled capacitor Ctot, and thus the voltage across the terminals of each modeled voltage source, can be decreased or increased.
In the prior art, there is thus an equivalent configuration for the set 6 of submodules of the MMC as shown in FIG. 4. In this figure, the converter is a converter that is analogous to that described with reference to FIG. 1, and in which each arm is replaced by its model. In addition, each phase line is associated with a current igi and with a voltage vgi (where i gives the number of the leg).
In this example, each of the modeled voltage sources has a voltage vmxi across its terminals, and each modeled capacitor Ctot has a voltage vcΣxi across its terminals (where x specifies whether the arm is upper or lower and i gives the number of the leg). It may also be observed that it is possible to consider the MMC as having an imaginary AC portion and an imaginary DC portion (for input or output depending on whether the converter is configured to convert AC energy into DC energy or vice versa), in which the variation of the total energy stored in the capacitors of the submodules is equal to the difference between the power entering the converter and the power leaving it.
Converters of the voltage source converter (VSC) type are known that possess a station capacitor connected in parallel with the DC power supply network. The drawback of such a parallel capacitor is that it does not enable the converter to be decoupled from the voltage of the DC power supply network. In addition, that type of converter requires the use of numerous filters in order to obtain acceptable converted signals.
In addition, the inertia of the DC power supply network depends on its capacitance, such that a large capacitance increases the inertia of the DC power supply network. Thus, a large capacitance of the network, and thus a large inertia of the network, enables it to withstand disturbances better. Conversely, a small capacitance of the network, and thus a small inertia of the network, enables the voltage across the connection points between the converter and the DC power supply network to be regulated more easily and more accurately.
However, and unlike VSC type converters, MMC type converters do not have a station capacitor connected in parallel and capable of having an influence on the stability of the DC power supply network. Modular multilevel converters thus present the advantage of providing decoupling between the total voltage across the capacitors of the submodules and the voltage of the DC power supply network. Nevertheless, merely varying power can lead to a large variation in the voltage of the DC power supply network.
MMC type converters are known in which control is not energy based. In such converters, in the event of a possible voltage difference appearing between the voltage across the capacitors of the arms and the voltage of the DC power supply network, the incoming power of the DC power supply network varies automatically in order to correct said voltage difference. Control is performed without an additional regulator since the exchanges of energy with the capacitors of the arms track the variations in voltage on the DC power supply network.
Nevertheless, all of the variables in converters of that type are not under control, which leads to a lack of robustness for the converter.
Converters are also known in which control is based on energy. In particular, the document entitled “Control of DC bus voltage with a modular multilevel converter” (by Samimi et al., PowerTech Conference 2015) is known and describes a modular multilevel converter having a system for controlling transfers of power in the AC portion, for controlling transfers of power in the DC portion, and for controlling the internal energy of the converter. Such a converter makes use of control that is energy based: controlling electrical variables of the DC power supply network and of the AC power supply network makes it possible to control the powers of those two networks. A difference between the powers of the DC and AC power supply networks leads to a reduction or to an increase in the energy stored in the capacitors of the submodules. Nevertheless, converters of that type are detrimental to decoupling between the voltages across the terminals of the capacitors of the submodules and the voltage of the DC power supply network. Furthermore, it does not make it possible to adapt effectively and in real time to voltage fluctuations on the DC power supply network.
Furthermore, known converters are not sufficiently robust, in particular concerning the contribution to stability of the DC power supply network.
In particular, controlling internal energy constitutes an additional degree of freedom, but no existing technique proposes a solution for regulating effectively the internal energy of the converter.
Existing solutions do not make it possible to make full use of the capacitances of MMC type converters in terms of controlling the internal energy of the converter jointly with controlling the stability of the DC network.